In general, hand grinders used for industrial machinery, shipbuilding, and other equipment are mainly used to smooth rough surfaces of primarily machined parts and portions to be painted, or to partially cut primary raw materials such as pipes, angles, iron plates, etc.
In other words, the hand grinder operates the rotary shaft at high speed by air pressure generated by the operation of an electric motor drive or a compressor, and at the same time, drives a grinding disc mounted on the rotary shaft to grind and polish a surface of an object formed of stone or metallic materials or trim or cut a portion or a corner that is not sufficiently machined, thereby finishing work.
As a structure to couple the grinding disc to the rotary shaft of a hand grinder, Korean Utility Model Publication Nos. 89-10365 and 96-26826 disclose that a grinding disc is inserted in a rotary shaft mounted in a grounder main body and fixed thereto by using a nut or by coupling a bolt to the rotary shaft.
Accordingly, in the structure, since a grinding disc is simply coupled to the grinder main body by using a bolt or nut, a coupling force therebetween is lowered due to a frictional force or fine vibration of the grinding disc rotating at high speed, making an operator feel uncomfortable, and actually the grinding disc is loosened during work, causing a safety accident. To prevent such an accident, a fixed-type cover is separately installed around the grinding disc.
Furthermore, when the grinding disc is replaced due to a long time use of a hand grinder, a bolt or nut portion may be hardened as foreign materials such as ground particles adhere thereto. Thus, the coupling of a bolt or nut is difficult so that the coupling is not secure, and thus a safety accident may occur.
Considering the above-described problems, Korean Patent Publication No. 2009-0074461 discloses a technology to provide a fixing holder on a rotary shaft of a grinder main body to couple a disc-integrated disc holder by screw coupling, thereby preventing a disc from being loosened from the fixing holder. Also, the technology enables the disc to be easily and rapidly coupled to or decoupled from the fixing holder through a break unit mounted on the grinder main body.
In the above structure, since the disc holder needs to be coupled to the fixing holder on the rotary shaft by screw coupling, it is inconvenient to release or recombine the disc holder during work. Also, since the separate break unit needs to be operated, it is quite inconvenient to an operator.
Furthermore, as another solution to address the problems, Korean Patent No. 0973157 discloses a technology which enables a disc holder mounted on a rotary shaft of a grinder main body to be simply coupled in a one-touch method so that the coupling/decoupling of the disc holder may be made easier and simultaneously the loosing of the disc holder may be prevented in advance by the operation of a safety key mounted in a stopper.
In the above-described structure, however, since foreign materials such as ground particles may adhere in a housing and the stopper, the operation of the safety key may be defective and also vibration may be generated during a grinding work.
Furthermore, the above structure needs lots of parts related to each other, and thus manufacturing costs are increased. Thus, the structure gives a considerable financial burden to work sites using a lot of consumables, for example, large shipyards.